1. Field of the Invention
The present invention relates to refractive imaging in general and x-ray refractive radiography in particular.
2. Discussion of Related Art
It is well known to use X rays for imaging the internal features of objects in those cases when the object is opaque in the visible optics domain, or when extremely high spatial resolution is necessary. Traditional x-ray imaging techniques are based on the absorption contrast, i.e., on the variation of the absorption factor of different parts of a sample. Therefore, the only way to increase a contrast of small objects in traditional x-ray images, is to increase the intensity of an x-ray beam. But this way is unacceptable in many cases. For example, strong x-ray beams cannot be used for visualization of the inner structure of integrated circuits because of their possible radiation damage, or for medical radiography for safety reasons. The refractive contrast, originating from the variation of the refractive indices of different parts of a sample, produces far more detailed images of the samples with small features. This type of x-ray imaging is commonly referred to as a phase contrast imaging (PCI). However, the direct beam, carrying practically no information about the object, if the latter is transparent to x rays, deteriorates the image, bringing additional noise into it. Therefore, the direct beam is undesirable.
One proposed way to suppress the direct beam is disclosed in U.S. Provisional Patent Application Serial No. 60/258,851, filed on Dec. 28, 2000, and U.S. Provisional Application Serial No. 60/272,354, filed Feb. 28, 2001, entitled xe2x80x9cDark-Field Phase Contrast Imagingxe2x80x9d and by Vladimir V. Protopopov, the entire contents of each of which are incorporated herein by reference. In each of those applications, several embodiments of an imaging system are disclosed. One embodiment is shown in FIGS. 1-2 where an x-ray tube 114 generates a beam 115 so that the long side of the focus 116 of the beam 115 is in the plane of incidence. The beam 115 is directed to a monochromator 118 that may be composed of two crystals 120, 122 that are well known in the art. The two crystals 120, 122 are selected so that they strongly disperse the beam 115 so as to generate highly parallel x-ray beams 100. In the embodiment of FIGS. 2 and 3, the object 102 is preferably no larger than several millimeters so that the object 102 is fully covered by the x-ray beam 100. Accordingly, there is no need to move the object 102 during imaging.
After the beam 100 interacts with the object 102, the beam 104 is directed to an analyzer 110 that suppresses the intensity of the original wave or beam 106 by several orders of magnitude in a manner as schematically shown in FIG. 8. The suppressed beam 106 and the refracted beam 108 are directed to the imaging plane 112 where a detector, such as an x-ray charge coupling device (CCD) 113, receives the beams. The detector then sends a signal to a processor (not shown) that generates an image that is formed on a display (not shown).
One embodiment of an analyzer 110 that can suppress the intensity of the beam 106 is shown in FIG. 6. In particular, the analyzer 110 of FIG. 6 is a specially designed multilayer mirror 124. The reflective coating of the x-ray multilayer mirror 124 is composed of many altering layers of materials with large and small atomic numbers. For instance, the layers 126 with large atomic numbers may be made of tungsten while the layers 128 with small atomic numbers may be made of boron-carbide, i.e., B4C. The thickness of the layers may differ, but they are typically of the order of 10 xc3x85-50 xc3x85. The interfacial roughness is equal to 5 xc3x85.
As described in xe2x80x9cX-Ray Multilayer Mirrors with an Extended Angular Range,xe2x80x9d by Protopopov et al., Optics Communications Vol. 158 (1998), pp. 127-140, the entire contents of which are incorporated herein by reference, it is possible to control the shape of the angular and spectral reflection curves by altering the thickness of the layers 126 and 128. Varying slightly the thickness of layers it is possible to make the partial reflected waves approximately counterphased at a specific grazing angle xcex8, so as to obtain as small reflection at this angle as possible. Moreover, the total reflection can be made even less if not only the phases of the partial waves are opposite each to another, but the coming and reflected waves produce interference pattern whose maxima at this particular angle coincide with the layers of heavy material, introducing additional absorption. Thus, it is possible to design a mirror with deep (the reflectivity of the order of 10xe2x88x922-10xe2x88x923) and narrow (several arc seconds) resonant gap in the angular reflection curve as shown in FIGS. 7a-b. The roles of reflection and absorption are clear from the solid and dashed curves, respectively, in FIG. 7a. In addition, the sensitivity of the scheme with respect to the refracted beams 108 is determined by the sharpness of the reflection curve around the resonant angle xcex8r. The sharpness of the gap in the reflection curve of the multilayer mirror 124 is sufficient to effectively detect small-contrast images.
If it is desired to image objects that are larger than 2 mm and have dimensions up to 150-200 mm, then a modified imaging system can be employed. This is advantageous for biological and medical applications. An embodiment of such an imaging system is shown in FIGS. 3-5. In this embodiment, the x-ray tube 114 works in the point projection mode. The width of the beam in the plane of incidence is limited by the x-ray tube focus, and is an order of magnitude less than in that for the imaging system of FIGS. 1-2. Consequently, the length of the mirror 110 in this direction may be much less than in the previous case.
As shown in FIG. 3, the x-ray tube 114 generates a beam 115 that is directed to the monochromator 118 that is composed of two crystals 120, 122 that are similar to those described previously with respect to the imaging system of FIG. 2. Again, the two crystals 120, 122 are selected so that they strongly disperse the beam 115 so as to generate highly parallel x-ray beams 100.
In the embodiment of FIGS. 3 and 4, the object 102 is preferably larger than the width of the x-ray beam 100. Accordingly, there is a need to move the object 102 relative to the detector 113 during imaging as shown in FIG. 5.
After the beam 100 interacts with the object 102, the beam 104 is directed to an analyzer 110 that suppresses the intensity of the original wave or beam 106 by several orders of magnitude in a manner as schematically shown in FIG. 8. The suppressed beam 106 and the refracted beam 108 are directed to the imaging plane 112 where a detector, such as an x-ray charge coupling device 113, receives the beams. The detector then sends a signal to a processor (not shown) that generates an image that is formed on a display (not shown). The analyzer 110 preferably has a structure that is similar to that as the analyzer 110 used in the imaging system of FIGS. 1-2.
As shown in FIG. 5, the object 102 is scanned in the plane of incidence in the direction transversal to the x-ray beam 100, so that each moment of time only a small fraction of the object is investigated. During each moment of time t the detector signal can be described by the matrix uij(t), where i and j are the ordinal numbers of its sensitive elements. The signals corresponding to the same row j but different column i differ each from another only by the time delay equal to ixcfx84, where xcfx84 is the time interval during which the object is shifted by a distance equal to a single detector element. Therefore, it is possible to average the signals from different columns if only take into account the delay. Such an averaging will raise the sensitivity and signal-to-noise ratio because the noise in the channels is uncorrelated. The time t corresponds to the first discrete coordinate of the image k by the formula t=kxcfx84, while the second discrete coordinate of the image is the row number j. Thus, the averaged discrete image can be written in the following form:             v      kj        =                            1          m                ⁢                              ∑                          i              =              1                        m                    ⁢                      xe2x80x83                    ⁢                                    u              ij                        ⁡                          (                              t                -                                  i                  ⁢                                      xe2x80x83                                    ⁢                  τ                                            )                                          =                        1          m                ⁢                              ∑                          i              =              1                        m                    ⁢                      xe2x80x83                    ⁢                                    u              ij                        ⁡                          [                              τ                ⁡                                  (                                      k                    -                    i                                    )                                            ]                                            ,
where m is the number of columns in the detector.
One disadvantage of the imaging systems shown in FIGS. 1-5 is that the analyzer may not satisfy the parameters required for some imaging applications. For example, medical imaging often involves small variations of the refractive indices within an area to be imaged. In this instance, very narrow and deep valleys in the reflection curve are required in some areas of phase contrast imaging and in particular in medical radiography.
Accordingly, it is an object of the present invention to significantly suppress the intensity of a direct beam during phase contrast imaging while providing very narrow and deep valleys in the reflection curve.
One aspect of the present invention regards an imaging system that includes a radiation generator that generates a beam of radiation along a first direction and an object that receives the beam of radiation, wherein a first portion of the beam of radiation is transmitted through the object along the first direction and a second portion of the beam of radiation is refracted along a second direction. A Fabry-Perot-like analyzer that receives the first and second portions of the beam of radiation, the Fabry-Perot-like analyzer suppresses the intensity of the first portion of the beam of radiation and transmits the second portion of the beam of radiation. A detector system that receives from the Fabry-Perot-like analyzer the suppressed first portion of the beam of radiation and the transmitted second portion of the beam of radiation and generates an image of the object.
A second aspect of the present invention regards an imaging system that includes a radiation generator that generates a beam of radiation along a first direction and an object that receives the beam of radiation, wherein a first portion of the beam of radiation is transmitted through the object along the first direction and a second portion of the beam of radiation is refracted along a second direction. An analyzer that receives the first and second portions of the beam of radiation, the analyzer suppresses the intensity of the first portion of the beam of radiation and transmits the second portion of the beam of radiation, the analyzer generating a reflecting curve with multiple valleys or peaks. A detector system that receives from the analyzer the suppressed first portion of the beam of radiation and the transmitted second portion of the beam of radiation and generates an image of the object.
A third aspect of the present invention regards an analyzer that includes a first multilayer structure, a spacer material deposited on the first multilayer structure and a second multilayer structure deposited on the spacer material.
An advantage of each aspect of the present invention is to significantly suppress the intensity of a direct beam during phase contrast imaging while providing very narrow and deep valleys in the reflection curve.
Additional objects and advantages of the invention will become apparent from the following description and the appended claims when considered in conjunction with the accompanying drawings.